rename/deprecate block_size -> block_copy_size, improve docs

[Wumpf]

Checklist

• Run cargo clippy.
• Run cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections

• Very loosely related to Add .bits_per_pixel() to the TextureFormat to simplify buffer size calculation for each texture format. #4054

Description
I keep seeing people getting confused about block_size, so I made its name more explicit and fixed the docs a bit (for instance the link in there was broken)

[teoxoy]

texel_block_size has been renamed to texel_block_copy_footprint in the spec which is a lot clearer in what it means. Can we use that instead?

gpuweb/gpuweb#3922

[Wumpf]

Oh didn't notice, thank you! Let's go with that then and link to it ofc.
That's a pretty awkward name though imho. Also doesn't imply bytes -.-

[Wumpf]

actually.. linking it okay. But do we think anyone will ever find TextureFormat::block_copy_footprint if they're looking for a byte size of a texel?

[Wumpf]

Technically we are providing the informative value that is described in the spec as

The texel block memory cost of a GPUTextureFormat is the number of bytes needed to store one texel block. It is not fully defined for all formats. This value is informative and non-normative.

so from that pov we would need to call it block_memory_cost :/

[Wumpf]

moving (potential) discussion to Matrix

[teoxoy]

The texel block memory cost is only "informative and non-normative".
The texel block copy footprint is the useful value since it's needed to calculate some of the parameters of ImageDataLayout/ImageCopyBuffer.

[teoxoy]

LGTM